EJECTA EMPLACEMENT

One of the most characteristic, but poorly understood, aspects of the impact cratering process is the generation of ejecta deposits. The lack of understanding is due, in part, to the scarcity of ejecta at the majority of the known terrestrial impact structures. Observations of impact ejecta deposits on other planetary bodies provide a complementary data set with which to study the emplacement of impact ejecta. Understanding the mechanism of impact ejecta generation and emplacement is important as these deposits provide a natural method to sample the subsurface of planetary bodies. It is, therefore, critical to understand the depth of origin of ejecta materials from any particular impact site.

Proximal ejecta deposits are rare on Earth due to post-impact erosional processes, but are common on other planetary bodies. It is generally accepted that proximal ejecta deposits on airless bodies, such as the Moon, are emplaced via ballistic sedimentation. In this model, the ballistic emplacement of primary crater-derived ejecta results in secondary cratering and the incorporation of local material (so-called “secondary ejecta”), and considerable modification of the local substrate.